[postgis-users] unioning a buffer - bug?

[Kevin Neufeld]

Interesting.  Something sure sounds amiss.  Are the results null?  Is 
there anything usual occurring at the TopologyException location?  Is 
your dataset large?  Can you winnow it down to the few polygons that 
might be causing the problem (this may be just the polygons around the 
TopologyException location)?  If so, you could paste the representation 
of the geometries in PasteBin or something so we can help you isolate 
the problem.

Also, what version of GEOS do you have installed?
SELECT PostGIS_Full_Version();

Cheers,
Kevin

On 9/14/2010 6:03 AM, Lee wrote:
> Hi Kevin,
>
> I assumed I have no results because when I load one of the working 
> queries as a view (eg. select st_buffer(the_geom,0.1)) in QGIS, or in 
> ArcGIS, I see all my results as I would expect, but when I modify the 
> query to st_union(st_buffer()), all my parcels disappear and I have a 
> blank canvas.
>
> Also, with st_isvalid(st_union(st_buffer(the_geom,0.1))) returning 
> null (neither t nor f) I guess I just assumed.  Anyways, results for 
> your suggestions are inline, below.
>
> Thanks for your help. Any further ideas?
>
> Lee.
> ------------------------------------------------------------------------
> *From:* Kevin Neufeld <kneufeld at refractions.net>
> *To:* postgis-users at postgis.refractions.net
> *Sent:* Tue, September 14, 2010 12:12:46 AM
> *Subject:* Re: [postgis-users] unioning a buffer - bug?
>
> How are you determining you have no results?
>
> Try:
> CREATE TABLE results AS
> SELECT st_union(st_buffer(the_geom,0.1)) as the_geom
> FROM base.current_assessment_parcel;
>
> NOTICE:  TopologyException: found non-noded intersection between 
> 586714 4.95189e+006, 586714 4.95189e+006 and 586714 4.95189e+006, 
> 586739 4.9519e+006 586714 4.95189e+006
> Query returned successfully with no result in 1078 ms.
>
>
>
> SELECT count(*) FROM results;
>
> 1
>
> SELECT ST_Summary(the_geom) FROM results;
>
> This returns blank.
>
>
> (snip)
>
> -- Kevin
>
> On 9/13/2010 7:07 AM, Lee wrote:
>> So after some googling I see similar problems, but no solutions posted.
>>
>> I am trying to union a buffer, but the query is returning no 
>> results.  Here is ultimately what I would like to achieve:
>>
>> select st_union(st_buffer(the_geom,0.1)) as the_geom from 
>> base.current_assessment_parcel
>>
>> Here are some troubleshooting steps I have taken to try narrow it down:
>>
>> select st_isvalid(the_geom) a from base.current_assessment_parcel 
>> group by a
>> TRUE
>>
>> select st_isvalid(st_buffer(the_geom,0.1)) a from 
>> base.current_assessment_parcel group by a
>> TRUE
>>
>> select st_isvalid(st_union(the_geom)) from base.current_assessment_parcel
>> TRUE
>>
>> select st_isvalid(st_union(st_buffer(the_geom,0.1))) from 
>> base.current_assessment_parcel
>> NOTICE:  TopologyException: found non-noded intersection between 
>> 586714 4.95189e+006, 586714 4.95189e+006 and 586714 4.95189e+006, 
>> 586739 4.9519e+006 586714 4.95189e+006
>> Total query runtime: 859 ms.
>> 1 row retrieved.
>> (returns blank record)
>>
>>  And just for fun
>> select st_isvalid(st_buffer(st_union(the_geom),0.1)) a from 
>> base.current_assessment_parcel
>> TRUE
>>
>> I guess my next step would be to try the snap to grid functions, but 
>> the first query above should work, shouldn't it?
>>
>> Any help appreciated. Thanks.
>> Lee
>>
>>
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>> postgis-users at postgis.refractions.net
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>>    
>
>
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